A nickel-hydrogen storage battery has been recently noted as a new alkaline storage battery because of its high capacity, at least twice as high as that of nickel-cadmium batteries, and its environmental friendly nature. With the spread of portable instruments, this nickel-hydrogen storage battery is expected to further increase its performance.
A hydrogen storage alloy, when incorporated in a negative electrode of the nickel-hydrogen storage battery, generally undergoes spontaneous oxidation to form an oxide layer on its surface. Accordingly, a hydrogen storage alloy electrode fabricated from such a hydrogen storage alloy, when used as a negative electrode of the nickel-hydrogen storage battery, presents a problem of low initial battery capacity that is attributed to the initial low activity of hydrogen storage alloy.
A method has been recently proposed, for example, in Japanese Patent Laying-Open No. Hei 5-225975, which immerses a hydrogen storage alloy in an acid solution, such as a hydrochloric acid solution, to remove an oxide layer formed on its surface.
This method contemplates to remove an oxide layer from a surface of the hydrogen storage alloy by immersing it in the acid solution. However, nickel and cobalt hardly elute into the acid solution so that active sites as of metallic nickel (Ni) and cobalt (Co) appear on the hydrogen storage alloy surface.
Upon removal of the oxide layer by the above-described method, the active sites as of metallic nickel and cobalt appear on the hydrogen storage alloy surface, so that the initial discharge capacity is increased. The reduction of electrical contact resistance between alloy particles also results to increase the high-rate discharge capacity to a slight degree. However, the electrical contact resistance between alloy particles is still too high to achieve a marked improvement in high-rate discharge capacity. Also, the method is still insufficient to prevent the buildup of pressure in the battery and improve a charge-discharge cycle life of the battery.
It is an object of the present invention to provide a hydrogen storage alloy which, when fabricated into an electrode for alkaline storage batteries, can provide an excellent charge-discharge cycle life performance, prevent the buildup of battery's internal pressure during overcharge and improve high-rate discharge characteristics, and also to provide a method for production thereof.